At the present time, virtually all commercially printed copy is produced through the use of three basic types of printing. One type is a relief plate that prints from a raised surface. Another type, lithographic printing, is based on the immiscibility of oil and water wherein the oily material or ink is preferentially retained in the image area of a printing plate and the water or fountain solution is retained by the non-image area. The third type is gravure that prints from a depressed surface.
In gravure printing, depressions, known as cells, are fashioned with high resolution on an otherwise smooth metal printing surface. Ink is then supplied to the imagewise indented metal surface of the cylinder and the ink preferentially occupies the indentation cells. The ink-coated cylinder is then rolled against the printing media to effect the actual printing. The metal to be indented is typically, but not exclusively, copper. For subsequent protection of the indented printing surface, and to prolong the printing life of the printing surface, it may be coated with harder and more durable materials such as chromium.
Gravure printing plates or cylinders were traditionally prepared using etching techniques. In preparing such cylinders or plates for gravure printing, the copper printing surface is coated with a photosensitive gelatin to which a desired latent image is usually transferred by exposure to light through a halftone positive screen in conjunction with a film carrying a continuous tone positive image. The latent image is then developed and etched into the copper surface by methods well known in the art to form an intaglio image therein.
Prints made from such cylinders and plates by this traditional method have been found objectionable in that the edges of depicted objects, and particularly the edges of printed letters or numerals, are frequently jagged or saw-toothed in outline and appear fuzzy rather than sharp and smooth as is desirable.
A variety of methods have since been developed for fashioning the cells on the cylindrical printing surface. The most standard of these at this time is electromechanical indentation with a diamond stylus. The method comprises the following steps:    (a) opto-electronically scanning the original by means of an optical illumination and scanning system which includes means for placing the original into focus;    (b) conversion of the light signals obtained during scanning of the original into electrical signals which reproduce the intensity of the light signal and then processing the electrical signals in an electronic computer;    (c) engraving the printing form with a graving tool which is controlled by the electrical signals thus produced.
A number of alternative means have been developed more recently, such as electron beam engraving. Direct laser engraving has also been proposed. There are numerous potential workflow and efficiency advantages to such direct imagewise structuring of the gravure plate using digitally controlled beams to remove some of the constituent material. Clearly one of these is the obviating of the mask preparation step and associated costs. However, to the extent that metal is being engraved, the power requirements tend to be very high. This problem, along with concerns regarding the management of the debris and other resulting residues created in the process, render this generic approach largely unattractive.
Another category of relief printing plates, sleeves and cylinders may be prepared by coating the blank, unprocessed plate, sleeve or cylinder element with a photosensitive polymer. The required printing relief, either in the form of a gravure element or a flexographic element, may be obtained by Imagewise exposure of the photopolymer layer, either on negative-acting or positive-acting form and then developing the exposed element in a suitable developer. The drawback with this approach, as applied in particular to gravure or intaglio printing in general, is that the photopolymers cannot compare with the traditionally employed metals for hardness and durability. This results in limited run-length and defeats one of the traditional differentiating strengths of gravure as a technology. Additionally, the photopolymer tends to be scratched by the doctor blade during use. This results in unacceptable print quality.
One approach, described in U.S. Pat. No. 6,048,446 (Michaelis), is to address this shortcoming by proceeding through all the lithographic steps as described above, but to then plate gravure material in the areas where photopolymer has been removed.
As a result of more recent advances in the field of lithography, there have been renewed proposals for the use of various forms of resist to be used as screens though which to chemically etch the indentations. As has been demonstrated by the semiconductor industry, the level of sophistication and resolution obtained in resist-based etching is easily capable of providing the required cell resolution.
While chemical engraving has tended to be associated with the traditional photographic methods described earlier, gravure cylinders can in fact be produced using photo-resists exposed on laser imaging systems. Thus, chemical engraving may be employed in combination with the latest pre-press technology as an alternative to electromechanical engraving. An example of this approach is the use of a laser to directly image a light-sensitive photopolymer resist using digital image data, followed by more traditional chemical etching to produce a gravure cylinder. This approach offers high speed of engraving together with the obvious attraction of being able to employ existing chemical engraving equipment lines. However, this approach has to date been based on rather expensive lasers of visible wavelengths. Along with this goes the inherent sensitivity of the coating media to ambient light, necessitating the use of amber or red light working conditions. Furthermore, the coating media employed tends to have a short shelf life.
Affordable infrared laser diodes or diode arrays with a very practical power output are now commercially available and can be used to form a mask image on top of a gravure printing element. The use of infrared wavelengths also inherently addresses the ambient light limitations of previous methods. The image to be developed can be translated into digital information and the digital information used to modulate the laser light for imaging. The laser light may be modulated, either within the laser or via a separate modulator, while being scanned across the media element.
Against this background there have been proposals for the preparation of gravure media elements employing a mask that is photo-imageable at wavelengths matching those produced by high efficiency laser diodes and diode arrays, such as those employed in commercial digital plate-making machines. However, the media suggested for use in these proposals is positive-working and suffers from the shortcoming that it has to be developed in a high pH developer. The basic positive-working approach suggested by these proposals also leads to operational problems with handling-induced printing artifacts, particularly in the specific case of gravure plates.
The need therefore remains in industry for a method to obtain a gravure printing element using digital imaging technology based on affordable commercial diode lasers and laser arrays and employing benign chemicals in the masking procedure. Particularly advantageous would be a method and apparatus that could reduce the amount of handling by integrating many of the gravure etch masking steps.